As is well known in the gas turbine engine technology, the cooling of the turbine blades, particularly, the first stage turbine, is extremely important not only to preserve the integrity of the blade structure but to also attain high engine performance by operating the turbine at optimum temperature levels. It is abundantly important in this environment to maximize the use of cooling air to avoid utilizing more air than is necessary so as to lessen the overall penalty that is attendant the use of engine air for purposes other than generating thrust or horsepower. Inasmuch as the gas turbine engine operates at higher efficiencies by operating the first stage turbine at higher temperatures and the trend for future engines is to increase turbine inlet temperatures so as enhance engine efficiency and thereby reduce fuel consumption, the engine designer is faced with the problem of increasing turbine inlet temperature while at the same time attempting to reduce the amount of cooling air or at the very least to optimize its use.
As is well known, one method of optimizing the use of cooling air is to employ metering devices to restrict the flow entering into the roots of each of the blades. Typically, these flow restrictive or metering devices are comprised of an extra sheet metal component that is welded or brazed to the bottom of the blade. An example of a metering device that is bolted to the root of a stator blade is exemplified in U.S. Pat. No. 3,706,508 granted to Moskowitz, et al on Dec. 19, 1972 and entitled "Transpiration Cooled Turbine Blade with Metered Coolant Flow".
United Kingdom Patent Application No. 2 225 063 A published for Ulrich Radons on May 23, 1990 entitled "Turbine Cooling Arrangement" discloses an insert that is bonded to the blade base for flowing coolant into the rotor blades. Other patents that, while not necessarily teach metering means, but relate to means for feeding coolant to the turbine rotor blades are U.S. Pat. No. 4,767,261 granted to Godfrey et al on Feb. 12, 1974 entitled "Cooled Vane" that utilizes a baffle plate internally of the vane; U.S. Pat. No. 3,791,758 granted to Jenkinson on Feb. 12, 1974 entitled "Cooling of Turbine Blades" that include divergent walls for defining a diffuser for leading coolant to the root of the blades; and U.S. Pat. No. 4,626,169 granted to Hsing et al on Dec. 2, 1986 entitled "Seal Means for a Blade Attachment Slot of a Rotor Assembly" that provides a baffle that leads coolant to the rotor blades.
We have found that we can attain a more efficient use of cooling air and eliminate the extra component parts that were heretofore necessary for metering coolant with a consequential improvement in the castibility of the blade, ease of fabrication and assembly by eliminating the brazing or welding operation, and eliminate the need to inventory the component parts. While this results in lowering costs, it does have the disadvantage of slightly increasing weight.
However, there is a distinct advantage when utilizing this invention in cast film cooled, high efficiency turbine blade designs. The pressure of the tangential onboard injectors (known as TOBI) that serves to transmit the cooling air in the rotating machinery to the roots of the turbine blades is determined on the blade's outflow requirements and airfoil root leading edge stagnation pressure. This, typically, provides higher than required pressure air to the remaining portion of the blade. Therefore, to maintain acceptable flow levels for main body film cooling and trailing edge flow restricting features,(for example, crossover and film holes) these holes must be sized relatively small. By decreasing this pressure in these areas by use of the metering valve, these flow restricting features can be enlarged without increasing flow. The advantage of being able to increase the size of the holes enhances the castability of the blade and the film effectiveness.
In actual tests of blades employing this invention, the inventive meter plate increased the size of the trailing edge crossover holes to approximately 30% larger than heretofore known designs. In addition, the design made it possible to add additional film cooling holes. In this configuration that was tested 3 extra film cooling holes were added in each of the rows of film cooling holes.